1. Field of the Invention
The present invention relates generally to integrated circuits, and more particularly to integrated circuit fabrication processes and structures.
2. Description of the Background Art
Various non-silicon-based devices are being manufactured for use in communications and other applications. Such devices are often sensitive to contamination or to the atmosphere in which they operate, and hence it is desirable for them to operate in a controlled atmosphere. Examples of these atmosphere-sensitive non-silicon-based devices include surface acoustic wave (SAW) devices, electro-optic modulators, acoustic-optic devices, and so on.
For example, let us consider SAW devices in more detail. SAW devices are often used in communication devices, such as, for instance, radio frequency (RF) filters in mobile phone handsets and communication networks. SAW devices utilize waves that propagate along the surface (or near surface) of a substrate. As used herein, SAW devices include those that utilize piezoelectrically-coupled Rayleigh waves and may also include those that utilize non-Rayleigh (skimming or “leaky”) waves. A typical SAW filter includes input and output transducers formed on a non-silicon-based piezoelectric substrate, such as, for example, lithium tantalate, lithium niobate, or single crystal quartz. The transducers may be metallic electrodes, for example, interleaved aluminum fingers. As an example of the size of a typical SAW device, one operating at 2.5 GHz may have a minimum feature size of approximately 0.4 microns for the aluminum fingers of the transducers.
One problem encountered with SAW devices is that the regions of the device where the acoustic waves are present can be very sensitive to the presence of surface contaminants that alter the wave velocities and consequently degrade the device performance. Even a monolayer of contaminant on the surface of the crystal can noticeably alter the device performance. Also, it is desirable for the SAW devices to operate in a low pressure (near vacuum) atmosphere, rather than in atmospheric air. Operating in such a low pressure atmosphere can decrease the viscous damping of the acoustic waves. Another problem associated with SAW devices is that a change in acoustic wave velocity is temperature dependent. In other words, a temperature change can change the velocity of the acoustic waves. This temperature dependence effectively limits the operable temperature range of SAW devices.